U.S. patent application number 13/974883 was filed with the patent office on 2014-12-25 for trapped burned gas fraction control for opposed-piston engines with uniflow scavenging.
This patent application is currently assigned to Achates Power, Inc.. The applicant listed for this patent is Achates Power, Inc.. Invention is credited to Nishit Nagar, Donovan M. Quimby.
Application Number | 20140373815 13/974883 |
Document ID | / |
Family ID | 52109872 |
Filed Date | 2014-12-25 |
United States Patent
Application |
20140373815 |
Kind Code |
A1 |
Nagar; Nishit ; et
al. |
December 25, 2014 |
Trapped Burned Gas Fraction Control for Opposed-Piston Engines with
Uniflow Scavenging
Abstract
A trapped burned gas fraction is controlled in a two-stroke
cycle opposed-piston engine with uniflow scavenging by adjusting an
external EGR setpoint in real time. The adjusted setpoint is used
to control EGR flow in the engine's air handling system.
Inventors: |
Nagar; Nishit; (San Diego,
CA) ; Quimby; Donovan M.; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Achates Power, Inc. |
San Diego |
CA |
US |
|
|
Assignee: |
Achates Power, Inc.
San Diego
CA
|
Family ID: |
52109872 |
Appl. No.: |
13/974883 |
Filed: |
August 23, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13926360 |
Jun 25, 2013 |
|
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13974883 |
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Current U.S.
Class: |
123/51B |
Current CPC
Class: |
F02M 26/23 20160201;
F02B 29/0418 20130101; F02D 2200/0411 20130101; F02D 41/0007
20130101; F02D 2200/0402 20130101; F01B 17/02 20130101; Y02T 10/30
20130101; F02D 41/0072 20130101; Y02T 10/34 20130101; F02M 35/1038
20130101; Y02T 10/40 20130101; F02M 26/05 20160201; Y02T 10/144
20130101; F02B 75/282 20130101; F02B 25/08 20130101; F02D 41/1458
20130101; F02B 75/28 20130101; F02D 43/00 20130101; F02B 29/0412
20130101; F01B 7/14 20130101; F02B 33/00 20130101; F02B 37/24
20130101; F02D 21/08 20130101; F02D 41/0062 20130101; Y02T 10/47
20130101; F02D 2041/141 20130101; F02M 35/10386 20130101; F02B
45/08 20130101; F02B 39/04 20130101; Y02T 10/12 20130101 |
Class at
Publication: |
123/51.B |
International
Class: |
F02B 75/28 20060101
F02B075/28; F02D 43/00 20060101 F02D043/00 |
Claims
1. A uniflow-scavenged opposed-piston engine equipped with an air
handling system, comprising: at least one cylinder with a bore,
longitudinally-spaced exhaust and intake ports, and a pair of
pistons disposed in opposition in the bore and operative to open
and close the exhaust and intake ports during operation of the
engine; a charge air channel to provide charge air to at least one
intake port; an exhaust channel to receive exhaust gas from at
least one exhaust port; an exhaust gas recirculation (EGR) loop
having a loop input coupled to the exhaust channel and a loop
output coupled to the charge air channel; and, a control
mechanization operable to: determine a value of a trapped air
handling parameter and to adjust the value of the trapped air
handling parameter in response to a rate of EGR flow in the EGR
loop; and adjust the rate of EGR flow in the EGR loop based on the
adjusted value of the trapped air handling parameter.
2. The opposed-piston engine of claim 1, in which the control
mechanization is operable to adjust the rate of EGR flow by
operating a valve to increase or decrease exhaust gas flow through
the EGR loop.
3. The opposed-piston engine of claim 2, in which the trapped air
handling parameter is trapped burned gas fraction and the control
mechanization is operable to: determine a desired trapped burned
gas fraction value for a current engine operating state; determine
a % EGR ratio defined by % E G R = W egr W air + W egr ##EQU00008##
in which W.sub.egr is a mass flow rate of EGR gas in the EGR loop
and W.sub.air is a mass flow rate of air into the charge air
channel; determine an error value based upon a difference between a
desired % EGR ratio and a measured % EGR ratio; and, adjust EGR
flow by operating a valve in the EGR loop in response to the error
value.
4. The opposed-piston engine of claim 2, in which the control
mechanization is operable to correct the value of the trapped air
handling parameter based upon a trapped temperature parameter.
5. The opposed-piston engine of claim 4, in which the trapped air
handling parameter is trapped burned gas fraction and the control
mechanization is operable to: determine a desired trapped burned
gas fraction value for a current engine operating state; determine
a % EGR ratio defined by % E G R = W egr W air + W egr ##EQU00009##
in which W.sub.egr is a mass flow rate of EGR gas in the EGR loop
and W.sub.air is a mass flow rate of air into the charge air
channel; determine an error value based upon a difference between a
desired % EGR ratio and a measured % EGR ratio; and, adjust EGR
flow by operating a valve in the EGR loop in response to the error
value.
6. A method of operating an opposed-piston engine, comprising:
generating exhaust gas in at least one ported cylinder of the
engine; transporting exhaust gas from an exhaust port of the ported
cylinder through an exhaust channel; recirculating a portion of the
exhaust gas from the exhaust channel; pressurizing fresh air;
mixing recirculated exhaust gas with the pressurized fresh air to
form charge air; pressurizing the charge air; providing the charge
air to an intake port of the ported cylinder; determining a value
of a trapped air handling parameter; adjusting the value of the
trapped air handling parameter in response to a rate of EGR flow in
the EGR loop; and adjusting the rate of EGR flow in the EGR loop
based on the adjusted value of the trapped air handling
parameter.
7. The method of claim 6, in which adjusting the rate of EGR flow
includes operating a valve to increase or decrease exhaust gas flow
through the EGR loop.
8. The method of claim 7, in which the trapped air handling
parameter is trapped burned gas fraction and determining a value
includes: determining a desired trapped burned gas fraction value
for a current engine operating state; determining a % EGR ratio
defined by % E G R = W egr W air + W egr ##EQU00010## in which
W.sub.egr is a mass flow rate of EGR gas in the EGR loop and
W.sub.air is a mass flow rate of air into the charge air channel;
determining an error value based upon a difference between a
desired % EGR ratio and a measured % EGR ratio; and, adjusting EGR
flow by operating a valve in the EGR loop in response to the error
value.
9. The method of claim 7, in which the control mechanization is
operable to correct the value of the trapped air handling parameter
based upon a trapped temperature parameter.
10. The method of claim 9, in which the trapped air handling
parameter is trapped burned gas fraction and determining a value
includes: determining a desired trapped burned gas fraction value
for a current engine operating state; determining a % EGR ratio
defined by % E G R = W egr W air + W egr ##EQU00011## in which
W.sub.egr is a mass flow rate of EGR gas in the EGR loop and
W.sub.air is a mass flow rate of air into the charge air channel;
determining an error value based upon a difference between a
desired % EGR ratio and a measured % EGR ratio; and, adjusting EGR
flow by operating a valve in the EGR loop in response to the error
value.
11. A uniflow-scavenged opposed-piston engine, comprising: at least
one cylinder with a bore, longitudinally-spaced exhaust and intake
ports, and a pair of pistons disposed in opposition in the bore and
operative to open and close the exhaust and intake ports during
operation of the engine; a charge air channel to provide charge air
to at least one intake port; an exhaust channel to receive exhaust
gas from at least one exhaust port; an exhaust gas recirculation
(EGR) loop having a loop input coupled to the exhaust channel and a
loop output coupled to the charge air channel; and, a control
mechanization operable to: determine a trapped burned gas fraction
value for a current engine operating state; determine a desired
setpoint for EGR flow in the EGR loop for the trapped burned gas
fraction value; compare the desired setpoint with an actual rate of
EGR flow in the EGR loop; and, adjust the position of an EGR valve
in the EGR loop based on the comparison.
12. The opposed-piston engine of claim 11, in which the control
mechanization is operable to adjust the actual rate of EGR flow in
the EGR loop by adjusting the position of the EGR valve.
13. The opposed-piston engine of claim 12, in which the control
mechanization is operable to correct the trapped burned gas
fraction based upon a trapped temperature value.
14. The opposed-piston engine of claim 13, in which the control
mechanization is operable to: determine a % EGR ratio defined by %
E G R = W egr W air + W egr ##EQU00012## in which W.sub.egr is a
mass flow rate of EGR gas in the EGR loop and W.sub.air is a mass
flow rate of air into the charge air channel; determine an error
value based upon a difference between a desired % EGR ratio and a
measured % EGR ratio; correct the desired % EGR ratio based on the
error; and, adjust EGR flow by adjusting the position of the EGR
valve based on the corrected desired % EGR.
15. The opposed-piston engine of claim 12, in which the control
mechanization is operable to: determine a % EGR ratio defined by %
E G R = W egr W air + W egr ##EQU00013## in which W.sub.egr is a
mass flow rate of EGR gas in the EGR loop and W.sub.air is a mass
flow rate of air into the charge air channel; determine an error
value based upon a difference between a desired % EGR ratio and a
measured % EGR ratio; correct the desired % EGR ratio based on the
error; and, adjust EGR flow by adjusting the position of the EGR
valve based on the corrected desired % EGR.
Description
PRIORITY
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 13/926,360, filed Jun. 25, 2013, which is
incorporated herein by reference.
RELATED APPLICATIONS
[0002] This application contains subject matter related to that of
the following commonly-assigned applications: U.S. application Ser.
No. 13/068,679, filed May 16, 2011, published as US 2011/0289916 on
Dec. 1, 2011; PCT application US2013/026737, filed Feb. 19, 2013;
U.S. application Ser. No. 13/782,802, filed Mar. 1, 2013; and U.S.
application Ser. No. 13/891,622, filed May 10, 2013.
BACKGROUND
[0003] The field is two-stroke cycle internal combustion engines.
Particularly, the field relates to uniflow-scavenged,
opposed-piston engines with air handling systems that provide
pressurized charge air for combustion, and that process the
products of combustion. In some aspects, such air handling systems
recirculate and mix exhaust gas with the pressurized charge air in
order to lower combustion temperatures.
[0004] A two-stroke cycle engine is an internal combustion engine
that completes a power cycle with a single complete rotation of a
crankshaft and two strokes of a piston connected to the crankshaft.
One example of a two-stroke cycle engine is an opposed-piston
engine with one or more cylinders, in which two pistons are
disposed in opposition in the bore of each cylinder for
reciprocating movement in opposing directions. Each cylinder has
longitudinally-spaced inlet and exhaust ports that are located near
respective ends of the cylinder. Each of the opposed pistons in the
cylinder controls one of the ports, opening the port as it moves to
a bottom center (BC) location, and closing the port as it moves
from BC toward a top center (TC) location. One of the ports
provides passage for the products of combustion out of the bore,
the other serves to admit charge air into the bore; these are
respectively termed the "exhaust" and "intake" ports. In a
uniflow-scavenged opposed-piston engine, charge air enters a
cylinder through its intake port and exhaust gas flows out of its
exhaust port, thus gas flows through the cylinder in a single
direction ("uniflow")--from intake port to exhaust port. The flow
of gas is referred to as the "gas exchange" process. The gas
exchange process occurs during that part of the cycle when the
intake and exhaust ports are open. For each cylinder of the engine,
gas exchange starts at the first port opening of a cycle and stops
at the last port closure of the cycle.
[0005] In FIG. 1, a uniflow-scavenged, two-stroke cycle internal
combustion engine is embodied by an opposed-piston engine 49 having
at least one ported cylinder 50. For example, the engine may have
one ported cylinder, two ported cylinders, three ported cylinders,
or four or more ported cylinders. Each ported cylinder 50 has a
bore 52 and longitudinally-spaced exhaust and intake ports 54 and
56 formed or machined in the cylinder wall, near respective ends of
the cylinder. Each of the exhaust and intake ports 54 and 56
includes one or more circumferential arrays of openings in which
adjacent openings are separated by a solid bridge. In some
descriptions, each opening is referred to as a "port"; however, the
construction of a circumferential array of such "ports" is no
different than the port constructions shown in FIG. 1. In the
example shown, the engine 49 further includes two crankshafts 71
and 72. The exhaust and intake pistons 60 and 62 are slidably
disposed in the bore 52 with their end surfaces 61 and 63 opposing
one another. The exhaust pistons 60 are coupled to the crankshaft
71, and the intake pistons are coupled to the crankshaft 72.
[0006] As the pistons 60 and 62 near TC, a combustion chamber is
defined in the bore 52 between the end surfaces 61 and 63 of the
pistons. Fuel is injected directly into the combustion chamber
through at least one fuel injector nozzle 100 positioned in an
opening through the sidewall of a cylinder 50. The fuel mixes with
charge air admitted into the bore through the intake port 56. As
the air-fuel mixture is compressed between the end surfaces it
reaches a temperature that causes combustion.
[0007] With further reference to FIG. 1, the engine 49 includes an
air handling system 51 that manages the transport of charge air
provided to, and exhaust gas produced by, the engine 49. A
representative air handling system construction includes a charge
air subsystem and an exhaust subsystem. In the air handling system
51, the charge air subsystem includes a charge source that receives
fresh air and processes it into charge air, a charge air channel
coupled to the charge air source through which charge air is
transported to the at least one intake port of the engine, and at
least one air cooler in the charge air channel that is coupled to
receive and cool the charge air (or a mixture of gasses including
charge air) before delivery to the intake port or ports of the
engine. Such a cooler can comprise an air-to-liquid and/or an
air-to-air device, or another cooling device. The exhaust subsystem
includes an exhaust channel that transports exhaust products from
exhaust ports of the engine for delivery to other exhaust
components.
[0008] With further reference to FIG. 1, the air handling system 51
includes a turbocharger 120 with a turbine 121 and a compressor 122
that rotate on a common shaft 123. The turbine 121 is coupled to
the exhaust subsystem and the compressor 122 is coupled to the
charge air subsystem. The turbocharger 120 extracts energy from
exhaust gas that exits the exhaust ports 54 and flows into an
exhaust channel 124 directly from the exhaust ports 54, or from an
exhaust manifold 125 that collects exhaust gasses output through
the exhaust ports 54. In this regard, the turbine 121 is rotated by
exhaust gas passing through it. This rotates the compressor 122,
causing it to generate charge air by compressing fresh air. The
charge air subsystem includes a supercharger 110. The charge air
output by the compressor 122 flows through a charge air channel 126
to a cooler 127, whence it is pumped by the supercharger 110 to the
intake ports. Charge air compressed by the supercharger 110 can be
output through a cooler 129 to an intake manifold 130. In this
regard, each intake port 56 receives pressurized charge air from
the intake manifold 130. Preferably, in multi-cylinder
opposed-piston engines, the intake manifold 130 is constituted of
an intake plenum that communicates with the intake ports 56 of all
cylinders 50.
[0009] In some aspects, the air handling system shown in FIG. 1 can
be constructed to reduce NOx emissions produced by combustion by
recirculating exhaust gas through the ported cylinders of the
engine. The recirculated exhaust gas is mixed with charge air to
lower peak combustion temperatures, which reduces production of
NOx. This process is referred to as exhaust gas recirculation
("EGR"). The EGR construction shown obtains a portion of the
exhaust gasses flowing from the port 54 during scavenging and
transports them via an EGR loop external to the cylinder into the
incoming stream of fresh intake air in the charge air subsystem.
Preferably, the EGR loop includes an EGR channel 131. The
recirculated exhaust gas flows through the EGR channel 131 under
the control of a valve 138 (this valve is also called the "EGR
valve").
[0010] In many two-stroke engines, combustion and EGR operation are
monitored and optimized based on various measurements related to
the amount of charge air delivered to the engine. For example, the
ratio of the mass of charge air delivered to a cylinder to the
reference mass of charge air required for stoichiometric combustion
in the cylinder ("lambda") is used to control NOX emissions over a
range of engine operating conditions. However, in a two-stroke
cycle opposed-piston engine with uniflow scavenging, port opening
times overlap for a portion of each cycle and some of the charge
air delivered to a cylinder through its intake port flows out of
the cylinder before the exhaust port is closed. The charge air
flowing out of the exhaust port during scavenging is not available
for combustion. Thus, a value of lambda based on charge air
delivered ("delivered lambda") to the intake port of a cylinder in
an opposed-piston engine with uniflow scavenging overstates the
amount of charge air actually available for combustion.
[0011] According to priority application Ser. No. 13/926,360, in a
two-stroke cycle opposed-piston engine with uniflow scavenging,
trapped lambda (.lamda..sub.tr) is estimated or calculated based
upon the charge air trapped in a cylinder by the last port to
close. In this regard, the last port to close can be either the
intake port or the exhaust port. Relatedly, the ratio of the mass
of charge air trapped in the cylinder by the last port to close
(hereinafter, "last port closing", or "LPC") to a reference mass of
charge air required for stoichiometric combustion in the cylinder
is referred to as "trapped lambda". Since it is the trapped charge
air that is available for combustion, a trapped lambda value
provides a more accurate representation of the combustion and
emission potentials of the engine than a delivered lambda value. A
method for determining trapped lambda (.lamda..sub.tr) is given in
priority application Ser. No. 13/926,360.
[0012] Other air handling parameters are used to control various
aspects of combustion and EGR operation in two-stroke engines and
determinations of their values are based on estimations or
calculations that include lambda. For example, burned gas fraction
(ratio of burned gas to in-cylinder mass) has a significant impact
on the combustion process and thus the emissions of a two-stroke
engine. Priority application Ser. No. 13/926,360 discloses a method
for determining trapped burned gas fraction (BF.sub.tr) using
trapped lambda. The trapped burned gas fraction is used to vary the
EGR flow rate using an EGR valve to minimize the error between the
actual and desired trapped burned gas fraction.
[0013] The trapped burned gas fraction provides an important
measure of the combustion process and thus of the emissions of an
opposed-piston engine. Control of the trapped burned gas fraction
can enable an air handling control mechanization to monitor and
adjust the combustion process and thereby control emissions as
engine operating conditions change. Control of an external burned
gas fraction alone does not always provide the degree of precision
as may be needed because there can be a significant difference
between in-cylinder trapped burned gas fraction and a burned gas
fraction based on external EGR. Therefore, in order to control
emissions, it is desirable to be able to control the trapped burned
gas fraction at all times.
[0014] Accordingly, there is a need to improve the accuracy of
trapped burned gas fraction control in uniflow-scavenged,
opposed-piston engines.
SUMMARY
[0015] A method is provided for controlling trapped burned gas
fraction in a two-stroke cycle opposed-piston engine with uniflow
scavenging by adjusting an external EGR setpoint in real time. The
setpoint is external in the sense that it relates to a condition or
element outside of (external to) any cylinder of the engine. In
some aspects, the external EGR setpoint is provided as an output
that the air handling system control mechanization produces
according to a current engine operating state. In this regard, the
trapped burned fraction is controlled based on determining a
portion of EGR useful for obtaining the desired trapped burned gas
fraction. This determination is based on air handling parameters
and an empirical scavenging model. Advantageously, the method
affords control of the trapped burned gas fraction in real
time.
[0016] In some aspects, the external EGR setpoint is called "%
EGR", which refers to a ratio of the mass flow rate of exhaust gas
through an EGR channel to the total mass flow rate of compressed
fresh air and exhaust gas through a charge air channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram of an opposed-piston engine equipped
with an air handling system with EGR and is properly labeled "Prior
Art".
[0018] FIG. 2 is a schematic drawing illustrating a control
mechanization for regulation of an air handling system in an
opposed-piston engine.
[0019] FIG. 3 is a control flow diagram showing a process for
evaluating and adjusting the numerical values of the air handling
control parameters.
[0020] FIG. 4 is a schematic diagram showing a control
mechanization that implements the evaluating and adjusting process
of FIG. 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] It is desirable to control the flow of charge air through
the charge air channel of a two-stroke cycle opposed-piston engine
with uniflow scavenging in order to maintain optimal control of
combustion and emissions in response to variations in the
operational state of the engine. Using the engine of FIG. 1 as a
basis, FIG. 2 shows a control mechanization for such an
opposed-piston engine, based on modifications and additions that
are useful for controlling the air handling system according to
this specification.
[0022] An example of a specific EGR loop construction for a
two-stroke cycle opposed-piston engine with uniflow scavenging is
the high pressure configuration illustrated in FIG. 2 (which is not
intended to be limiting). In this regard, a high pressure EGR loop
circulates exhaust gas obtained from a source upstream of the input
to the turbine 121 to a mixing point downstream of the output of
the compressor 122. In this EGR loop, the EGR channel 131 and the
EGR valve 138 shunt a portion of the exhaust gas from the exhaust
channel 124 into the charge air channel 126 where it is mixed with
compressed fresh air output by the compressor 122. Operation of the
valve 138 is controlled by the actuator 141 in response to an EGR
control signal. If no exhaust/air mixing is required, the valve 138
is fully shut and charge air with no exhaust gas component is
delivered to the cylinders. As the valve 138 is increasingly
opened, an increasing amount of exhaust gas is mixed into the
charge air. Conversely, from an open state, as the valve 138 is
increasingly closed, a decreasing amount of exhaust gas is mixed
into the charge air. This loop subjects the recirculated exhaust
gas to the cooling effects of the two coolers 127 and 129. If less
cooling is merited, the exhaust gas portion can be shunted around
the cooler 127 to the input of the supercharger 110; this
alternative subjects the exhaust gas portion to cooling by only the
charge air cooler 129. A dedicated EGR cooler to cool only exhaust
gas can be incorporated into the EGR channel 131, in series with
the valve 138, or in series with the output port of the valve 138
and the input to the supercharger 110.
[0023] As per FIG. 2, in most aspects the supercharger 110 is
coupled by a drive mechanism 111 to a crankshaft to be driven
thereby. The drive mechanism 111 can comprise a stepwise
transmission, or continuously variable transmission (CVT), device,
in which cases, charge air flow can be varied by varying the speed
of the supercharger 110 in response to a speed control signal
provided to the drive mechanism. Alternatively, the drive mechanism
111 can be a fixed gear device, in which case the supercharger 110
is continuously driven at a fixed speed. In such a case, charge air
flow can be varied by way of a shunt channel 112 that couples the
output of the supercharger 110 to its input. Provision of a bypass
valve 139 in the shunt channel 112 allows the charge air flow to be
varied by modulation of charge air pressure downstream of the
supercharger outlet. In some aspects, the valve 139 is operated by
an actuator 140 in response to a shunt control signal.
[0024] As seen in FIG. 2, a control mechanization to operate the
air handling system of a two-stroke cycle opposed-piston engine
with uniflow scavenging includes an ECU 149. Preferably, the ECU
149 is constructed to control charge air flow and the amount of
exhaust gas mixed with the pressurized charge air in response to
specified engine operating conditions by automatically operating
the valves 138 and 139 (and, possibly other valves), the
supercharger 110, if a multi-speed or variable speed device is
used, and the turbo-charger, if a variable-geometry device is used.
Of course, operation of valves and associated elements used for EGR
can include any one or more of electrical, pneumatic, mechanical,
and hydraulic actuating operations. For fast, precise automatic
operation, it is preferred that the valves be high-speed,
computer-controlled devices with continuously-variable settings.
Each valve has a state in which it is open (to some setting
controlled by the ECU 149) to allow gas to flow through it, and a
state in which it is closed to block gas from flowing through
it.
[0025] Methods for controlling the trapped burned gas fraction of a
two-stroke cycle opposed-piston engine with uniflow scavenging
(hereinafter, "the engine") use various parameters to calculate or
estimate magnitudes and ratios of elements of combustion trapped in
a cylinder of the engine by the last port closing of the cylinder.
In this regard, the "elements of combustion" include either or both
of constituents and products of combustion. For a better
understanding of these methods, an explanation of a number of air
handling parameters used to represent these elements is given with
reference to various elements of an air handling control
mechanization according to FIG. 2. All of the air handling
parameters in the following explanation have SI units unless
specified otherwise.
[0026] Air Handling Parameters
[0027] W.sub.air=Mass flow rate of fresh air in kg/s
[0028] W.sub.egr=Mass flow rate of EGR gas in kg/s
[0029] W.sub.sc=Mass flow rate of delivered charge air to a
cylinder in kg/s
[0030] W.sub.f=Commanded engine fuel injection rate in kg/s
[0031] M.sub.res=Mass of residuals in cylinder in kg
[0032] M.sub.tr=Mass of trapped cylinder gases at LPC in kg
[0033] M.sub.ret=Mass of delivered charge air retained in cylinder
in kg
[0034] M.sub.del=Mass of charge air delivered to the cylinder in
kg
[0035] M.sub.O.sub.2--.sub.tr=Mass of trapped oxygen at end of a
gas exchange process
[0036] m.sub.O2.sub.--.sub.air=Mass fraction of O.sub.2 in fresh
air
[0037] m.sub.O2.sub.--.sub.egr=Mass fraction of O.sub.2 in EGR
[0038] m.sub.O2.sub.--.sub.res=Mass fraction of O.sub.2 in cylinder
residuals
[0039] m.sub.O.sub.2--.sub.im=Mass fraction of O.sub.2 in intake
manifold
[0040] T.sub.comp.sub.--.sub.out=Compressor out temperature in
K
[0041] T.sub.egr=EGR temperature after cooler in K
[0042] T.sub.tr=Temperature of trapped charge in cylinder at LPC in
K
[0043] [O.sub.2].sub.im=Percent volumetric concentration of O.sub.2
in intake manifold
[0044] [O.sub.2].sub.egr=Percent volumetric concentration of
O.sub.2 in exhaust gas
[0045] [O.sub.2].sub.air=Percent volumetric concentration of
O.sub.2 in fresh air
( O 2 F ) s = Stoichiometric oxygen to fuel ratio ( A F ) s =
Stoichiometric air to fuel ratio ##EQU00001##
[0046] .gamma.=Ratio of specific heats
[0047] N=Number of cylinders
[0048] V.sub.d=Displacement volume per cylinder in m.sup.3
[0049] V.sub.tr=Displacement volume at LPC per cylinder in
m.sup.3
[0050] R=Gas constant of air J/Kg/K
[0051] R.sub.o2=Gas constant of oxygen in J/Kg/K
[0052] AFR.sub.s=Stoichiometric air fuel ratio for diesel
[0053] AFR.sub.g=Global air fuel ratio (ratio of fresh air to
fuel)
[0054] AFR.sub.tr=Trapped air fuel ratio (ratio of air in cylinder
to fuel)
[0055] P.sub.rail=fuel rail Pressure
[0056] Inj_time=Injection Timing
[0057] Definitions
[0058] Trapped lambda: a useful method for determining trapped
lambda is given in priority application Ser. No. 13/926,360 by:
.lamda. tr = ( N W f M O 2 _tr RPM 60 ) ( O 2 F ) s Eq 1
##EQU00002##
[0059] Burned gas is a gas composition that is the result of
stoichiometric combustion of fuel. This gas composition does not
have any oxygen molecules; typically, it comprises CO2, H2O, N2 and
other inert gases present in air.
[0060] Burned gas fraction is a ratio of burned gases to a
reference mass. A burned gas fraction of 1 indicates stoichiometric
combustion, implying that all the oxygen in the air has been used
up to convert fuel (C.sub.xH.sub.y) into CO.sub.2 and H.sub.2O. On
the other hand, a burned gas fraction of <1 indicates
non-stoichiometric combustion, implying that some the oxygen
remains in addition to the burned gas.
[0061] Trapped burned gas fraction is a ratio of burned gas trapped
in a cylinder at the end of the gas exchange process to the trapped
mass.
[0062] At the end of the gas exchange process, signified by LPC,
the trapped mass is equal to trapped air and trapped burned gases.
Thus, a trapped burned gas fraction can be calculated as
follows:
BF tr = ( M tr - W f N .lamda. tr AFR s 60 RPM ) M tr Eq 2
##EQU00003##
[0063] Another method of determining a trapped burned gas fraction
is given by Equation 35 in priority application Ser. No.
13/926,360.
[0064] Trapped Burned Gas Fraction Control Using % EGR: Air
handling control can be implemented using an air handling control
mechanization based on that illustrated in FIG. 2, in which the ECU
149 can be programmed to control operations of the air handling
system by methods illustrated by the diagrams of FIGS. 3 and 4. In
this regard, FIG. 3 shows a process for evaluating and adjusting
the numerical values of air handling control parameters. FIG. 4
schematically illustrates a preferred control mechanization that
implements the evaluating and adjusting process of FIG. 3.
[0065] Initially, the ECU 149 reads available engine sensors 200 so
as to determine current numerical values for air handling
parameters in the current state of engine operation. Using these
sensor values, the ECU 149 determines a current engine operating
state in terms of torque demand (load) and RPM and performs a
routine comprising a sequence of operations and calculations
corresponding essentially to Equations 1 and 2.
[0066] EGR Setpoint Determination: A desired trapped burned gas
fraction can be obtained directly by adjusting the EGR valve 138. A
desired trapped burned gas fraction can also be obtained by first
determining a desired % EGR setpoint from it and other operating
conditions and then adjusting the EGR valve 138. This method is
advantageous because, once a desired % EGR setpoint is determined
the ECU 149 can use it to regulate the EGR rate. The following
equations outline the method to determine a % EGR setpoint from a
trapped burned gas fraction setpoint.
[0067] The mass of EGR (M.sub.egr) required to meet a desired
trapped burned gas fraction can be calculated as follows:
M egr = BF tr M tr - M res BG res .eta. tr BG exh Eq 3
##EQU00004##
[0068] In Equation 3, BF.sub.tr is a desired target values obtained
from an empirical model and stored in or with the ECU 149, and
BG exh = ( AFR s + 1 ) ( AFR g + 1 ) Eq 4 BG res = 1 - AFR s (
.lamda. tr - 1 ) M tr W f 60 RPM N Eq 5 ##EQU00005##
[0069] Methods for determining M.sub.tr and BF.sub.tr are set forth
in in priority application Ser. No. 13/926,360.
[0070] Since M.sub.egr is now known, W.sub.egr can be calculated
by:
W egr = M egr ( N ) ( RPM ) 60 Eq 6 ##EQU00006##
[0071] Thus, the % EGR required to reach a desired BF.sub.tr can be
calculated by:
% E G R = W egr W air + W egr Eq 7 ##EQU00007##
[0072] EGR Control Method to Achieve Trapped Burned Gas fraction
Setpoint: The external EGR setpoint is controlled by adjusting the
position of the EGR valve 138 shown in FIG. 2. The ECU 149 reads
all the available engine sensors 200. Based on the sensor values
the EGR control method determines the current engine torque demand
(load) and RPM. This information is fed into a desired trapped
cylinder condition routine executed by the ECU 149. In this routine
the ECU 149 determines BF.sub.tr based on look up maps (tables)
that are indexed by engine torque and RPM to meet desired
performance and emission goals. The determined BF.sub.tr setpoint
is corrected for variations in trapped temperature based on another
map. These maps are pre-filled based on engine dynamometer testing
and stored in or with the ECU 149. The ECU 149 determines a desired
external % EGR and executes a method of controlling trapped burned
gas fraction. Representative embodiments of the method are
illustrated by the flowchart shown in FIG. 3 and the control
diagram of FIG. 4.
[0073] As per FIG. 3, a trapped burned gas fraction routine 300
accesses a map to determine a desired trapped burned gas fraction
value for the current engine operating state at step 302 and then,
in step 304, determines the desired mass flow rate of EGR that
meets the desired trapped burned gas fraction value. In step 306,
an actual (measured or calculated) EGR mass flow rate value is
compared to the desired EGR mass flow rate value. Preferably the
comparing process includes subtracting the desired value from the
measured value. If the absolute value of this difference is greater
than a threshold, the routine 300, in step 308, adjusts the
position of the EGR valve 138 to bring the absolute value to, or
below the threshold. The routine 300 cycles continuously during
engine operation. In some aspects, the cycle time of the routine
300 can exceed or equal the engine's cycle time. In yet other
aspects, the routine 300 can cycle on a per-cylinder basis.
[0074] An exemplary control mechanization with which actual trapped
burned gas fraction can be controlled is shown in FIG. 4. This
control mechanization 400 includes a trapped burned gas fraction
controller comprising a feed forward controller 410, a feedback
controller 412, and maps 416 and 418 of desired trapped burned gas
fraction and desired % EGR ratio, respectively. The feed forward
controller 410 outputs an EGR valve setpoint .THETA. based on a map
that is indexed by engine load and speed. This map is pre-filled
with empirical data based on engine dynamometer testing and stored
in or with the ECU 149. The map 416 outputs a desired burned gas
fraction setpoint (BF.sub.tr.sub.--.sub.sp) based on a table of
values that is indexed by engine load and speed. The map 418
outputs a desired EGR mass flow rate (W.sub.egr.sub.--.sub.des)
based on a table of values that is indexed by the desired burned
gas fraction setpoint produced by map 416. The maps 416 and 418 are
pre-filled with empirical data based on engine dynamometer testing
and stored in or with the ECU 149. At 420, the actual EGR mass flow
rate is determined by sensing and/or by calculation; see Equations
5 and 6 in priority application Ser. No. 13/926,360 for examples of
EGR mass flow measurement and calculation.
[0075] The adder 421 generates an error based on comparison of the
desired and actual % EGR ratios (step 306 of FIG. 3), and the
feedback controller 412 converts this error into a change required
in EGR flow (.DELTA.W.sub.EGR) to minimize the error. The feedback
controller 412 can be implemented with a PID controller, a gain
scheduled PID controller, or another non-linear controller such as
a sliding mode controller. An EGR valve actuator controller 423
converts the required change in EGR flow into an EGR valve
position. The EGR actuator controller 423 that translates EGR flow
change to an EGR actuator output (.DELTA..THETA.) can be
implemented as a PID controller or gain scheduled PID controller in
conjunction with an EGR model (physical or empirical). The EGR
actuator output from the actuator controller 423 is added (or
subtracted depending on the sign) to the output of the feed-forward
controller 410 at 424. A final EGR valve actuator command is then
sent via the ECU 149. The EGR valve actuator command is provided as
an EGR control signal to the EGR valve actuator 141.
[0076] Referring again to FIG. 4, in some aspects it may be
desirable to adjust the desired burned gas fraction setpoint based
upon estimation of the temperature of the trapped mass in the
cylinder at LPC. It can be calculated by an arithmetic unit 426 as
per Equation 42 of priority application 13/926,360. In this regard,
the unit 426 then compares actual trapped temperature T.sub.tr to a
predefined value of trapped temperature. The predefined value of
trapped temperature is determined based on engine dynamometer
testing and is stored in or with the ECU 149. If the actual trapped
temperature turns out to be greater than this predefined
temperature then the control mechanization 400 adjusts the desired
trapped burned gas fraction setpoint at 428 to minimize the impact
of trapped temperature on emissions. In this regard, adjustments to
the desired trapped burned gas fraction setpoint are made on the
basis of a map implemented by a look up table 427, which is indexed
by a trapped temperature error e.sub.T.sub.tr output by 426. The
values for this look up table can be determined by engine
dynamometer testing and stored in or with the ECU 149.
[0077] Although the embodiments illustrated and described herein
attribute actual parameter values based on conditions in the
manifolds 125 and 130 to the cylinders of the engine, it should be
evident to those skilled in the art that the principles involved
can be applied to the individual cylinders themselves, presuming
that cost and space permit placement and operation of relevant
sensors on one or more of the cylinders of a production engine.
Further, the desired parameter values are obtained by empirical
methods that map or synchronize those values to port closing times
for a cylinder of a uniflow scavenged, two-stroke cycle
opposed-piston engine running, for example, in a dynamometer.
[0078] Although air handling control has been described with
reference to a particular trapped parameter and adjustment of a
particular external condition, those of ordinary skill in the art
will realize that control of trapped burned gas fraction by
adjustment of an EGR flow or an EGR setpoint may be combined with
control of other trapped conditions by adjustment of other air
handling parameters.
[0079] Although air handling control has been described with
reference to an opposed-engine with two crankshafts, it should be
understood that these constructions can be applied to
opposed-piston engines with one or more crankshafts. Moreover,
various aspects of these constructions can be applied to
opposed-piston engines with ported cylinders disposed in
opposition, and/or on either side of one or more crankshafts
[0080] Accordingly, the patent protection accorded to the
above-described constructions is limited only by the following
claims.
* * * * *